In lithography, there is an ongoing desire to reduce the size of features in a lithographic pattern in order to increase the density of features on a given substrate area. In photolithography, the push for smaller features has resulted in the development of technologies such as immersion lithography and extreme ultraviolet (EUV) lithography, which are however rather costly.
A potentially less costly road to smaller features that has gained increasing interest is so-called imprint lithography, which generally involves the use of a “stamp” (often referred to as an imprint template) to transfer a pattern onto a substrate. An advantage of imprint lithography is that the resolution of the features is not limited by, for example, the emission wavelength of a radiation source or the numerical aperture of a projection system. Instead, the resolution is mainly limited to the pattern density on the imprint template.
Imprint lithography involves the patterning of an imprintable medium on a surface of a substrate to be patterned. The patterning may involve contacting a patterned surface of an imprint template with a layer of imprintable medium such that the imprintable medium flows around protrusions and into recesses in the patterned surface and is pushed aside by protrusions on the patterned surface. The recesses and protrusions define pattern features of the patterned surface of the imprint template. Typically, the imprintable medium is flowable as the patterned surface contacts the imprintable medium. Following patterning of the imprintable medium, the imprintable medium is suitably brought into a non-flowable or frozen state and the patterned surface of the imprint template and the patterned imprintable medium are separated. The substrate and patterned imprintable medium are then typically processed further in order to pattern or further pattern the substrate. The imprintable medium is typically formed from imprintable medium droplets on the surface of a substrate to be patterned.
In order to be able to accurately apply patterns to imprintable medium provided on a substrate, it is desirable to be able to accurately align the imprint template used to imprint that pattern with a specific area or part of the substrate. Partial or coarse alignment may take place before the pattern of the imprint template is imprinted into the imprintable medium. Final and fine alignment may take place when the pattern of the imprint template is imprinted into the imprintable medium, and before, for example, the pattern in the imprintable medium is fixed or frozen by exposure to UV radiation. Alignment commonly involves directing radiation onto or through two spaced apart gratings, one on or in the imprint template and one on or in the substrate. Diffraction of the radiation and/or the movement of fringes may be used in the alignment process.